Epistemology is the branch of philosophy that deals with the nature of knowledge and truth. It encompasses the study of the origin, nature, and limits of human knowledge. There are various ways people approach this task; the following categories originally reflected divisions among schools of philosophy in the seventeenth and eighteenth centuries, but are useful in categorizing certain approximate trends throughout its history:

(1) Rationalists (see rationalism) believe there are innate ideas that are not found in experience. These ideas exist independently of any experience we may have. They may in some way derive from the structure of the human mind, or they may exist independently of the mind. If they exist independently, they may be understood by a human mind once it reaches a necessary degree of sophistication

(2) Empiricists (see: empiricism, scientific method, philosophy of sciencenaive empiricism) deny that there exist any concepts that exist prior to experience. For them, all knowledge is a product of human learning, based on human perception. Perception, however, is a cause of concern since illusions, misunderstandings and hallucinations prove that perception does not always depict the world as it really is.

A problem for empiricists is the existence of mathematical theorems; their truths certainly do not depend on experience, and they can be known prior to experience. Empiricists rebut that all mathematical theorems are empty of cognitive content, as they only express the relationship of concepts to one another. Rationalists would hold that such relationships are indeed a form of cognitive content.

(3) The German philosopher Immanuel Kant is widely understood as having worked out a synthesis between these views. In Kant's view people certainly do have knowledge that is prior to experience, which is not devoid of cognitive significance. For example, the principle of causality. He held that there are a priori synthetic concepts.

People in all schools of thought agree that we have the capacity to think of questions that no possible appeal to experience could answer. For instance: Is there an end to time? Is there a God? Is the God of the philosophers the same as the Biblical God? Is there a reality beyond that which we can sense? Such questions are termed transcendental, as they seem to go beyond the limits of rational inquiry. In the 20th century logical positivists have declared such questions to be totally devoid of cognitive significance. Others disagree, and hold that only some metaphysical claims are devoid of cognitive significance however some may not be.

There is no consensus as to which epistemology will be the most productive in allowing human beings to have the most accurate understanding of the world. While not widely appreciated, all people use an epistemology, even if unconsciously. Thinking beings cannot understand and analyze ideas without first having a system to accept and analyze information in the first place, which we all do. All people - even children - possess rudimentary and undeveloped epistemologies. However, only those who study some philosophy and logic can begin to recognize how their own epistemologies work; only they can choose to change their epistemology, if they so wish.

Our analysis then will be dependent on the system we used to begin with.
One might wonder: What do I have to do, to be sure that I do have the truth? How can I be sure that my beliefs are true? Is there some sort of guarantee available to me -- some sort of criterion I might use, in order to decide, as rationally and as carefully as I possibly could, that indeed what I believe is true?

Suppose you thought your belief had been arrived at rationally. You used logic, you based your belief on observation and experiment, you conscientiously answered objections, and so forth. So you conclude that your belief is rational. Well if so, then your belief has at least some claim to be true. Rationality is a indicator of truth: if your belief is rational, then it is at least probably true. At the very least, the rationality of a belief is reason to think the belief is true.

A feature of belief is an epistemic feature if it is at least some indication that the belief is true.

Many of our beliefs do have lots of positive epistemic features; many of our beliefs are quite rational, quite justified, very probably true, highly warranted, and so on. But most of us, at least in some moments, don't want to rest content with just being rational. We don't want to have a rational belief that is, unfortunately, false. Because that's possible, right? I can be very conscious, careful, and logical in forming a belief, and so be rational in holding the belief; but it still might be false. So rationality isn't our ultimate ambition that we have for our beliefs.

Our ultimate ambition for our beliefs is knowledge. Because if I do know something, then not only am I justified, or rational, in a belief; because I have knowledge, I have the truth. So naturally, when we are thinking about the epistemic features of our beliefs, the big question is this: When do I have knowledge? When can I say that I have it? As I'm sure you are aware, there are some people who claim that we can't have knowledge; such people are called skeptics. More on that, of course, later.

Now I can describe to you the field of epistemology, which is also called the theory of knowledge. Here is a definition:

Epistemology includes the study of (1) what the epistemic features of belief, such as justification and rationality, each are (e.g., what justified belief is); (2) the origin or sources of such features (and thus the sources of knowledge); (3) what knowledge is, i.e., what epistemic features would make a true belief knowledge; (4) whether it is possible to have knowledge.

So, first, epistemologists spend a great deal of time concerning themselves with various epistemic features of belief, such as justification and rationality. And they write long articles and books trying to say just when beliefs are justified, or rational.

A second, related concern is where such epistemic features ultimately come from. If I say, for example, that my belief that Paris is the capital of France is justified, I can ask: Where did the justification for my belief come from? Probably at some point some reliable source told me that Paris is the capital of France; and that was enough to make me justified in adopting the belief. OK, then one, but only one, source of justification would be testimony, which is just a fancy word for what other people tell me. Another source of justification would be sense-perception. So epistemology asks: What are the ultimate sources of justification, rationality, or other epistemic features of belief? And that allows to answer a further question: What are the ultimate sources of knowledge?

Which brings us to the third topic studied by epistemologists, namely, what knowledge is. The question here isn't what we can know, or even what we do know. The question is: What would knowledge be, if we had it? A belief has to pass some sort of muster if it's to count as knowledge. So what features would a belief have to have, in order to be an actual piece of knowledge -- not just something that pretends to be knowledge, but which is actually knowledge?

Then, fourth, there is one of the more difficult topics of philosophy -- trying to answer, or otherwise deal with, the challenge that we cannot have knowledge. A number of philosophers -- not too many, but some -- have said that we cannot have knowledge. A lot of philosophers have said that it's very difficult to obtain knowledge; but they don't deny that we have it, or that we can have it. Not so many philosophers, however, have gone so far as to say that we have no knowledge at all, or (to say something even stronger) that it is impossible to have knowledge.